<html>
	<head>
		<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
	</head>
	<body>
	<form action="testing.php" method="POST">
		<select id="optionTest">
			<option value="1">so 1</option>
			<option value="2">so 2</option>
			<option value="3">so 3</option>
		</select>
		<input id="them_vao" value="1" name="them_vao">
		<input type="submit" value="dang">
	</form>
		<script type="text/javascript">
			$("#optionTest").change(function(){
				var optionTestVal = 1;
				$("#optionTest option:selected").each(function(){
					optionTestVal = $(this).val();
					$("#them_vao").val(optionTestVal);
				})
			}).change();
		</script>
	</body>
</html>